Abstract

We present two cases of hyperthyroid cardiomyopathy (CMP) caused by Graves’ disease in which multidetector computed tomography (MDCT) findings provided specific diagnostic clues, guiding the attending physician to a prompt diagnosis and allowing rapid initiation of therapy. We believe that a combination of diffuse thyroid enlargement, with decreased attenuation on non-enhanced chest CT, thymic hyperplasia and cardiomegaly (i.e. predominantly right heart failure with enlargement of the left atrium) are suspicious CT findings of hyperthyroid CMP.

1. Introduction

Diagnosis of hyperthyroid cardiomyopathy (CMP) has been mainly established on the basis of clinical and laboratory findings (1-5). However, as the field of view of routine chest computed tomography (CT) includes both thyroid glands and heart, multidetector CT (MDCT) has a potential to provide important clues to the presence of hyperthyroid CMP, even if clinical information is not available. We present two cases of hyperthyroid CMP, in which MDCT findings suggested the diagnosis, prior to detailed clinical work-up.

Figure 1. A 47-year-old male with exertional dyspnea and atrial fibrillation. A and B, Diffuse thyroid enlargement (arrowheads on A and B) with a decreased attenuation (HU = 71) was noted on non-enhanced chest CT. Note the loss of the normal high attenuation in the thyroid glands (B). C, Mild thymic hyperplasia (arrowheads) and enlargement of main pulmonary artery (arrows) are demonstrated on contrast enhanced chest CT, at the level of bifurcation of the main pulmonary artery. Right pleural effusion is also noted. D, Mild thymic hyperplasia (arrowheads) is evident on a coronal CT image. E, Enlargement of the left and right atria and right pleural effusion are demonstrated on contrast enhanced chest CT, at the level of the left atrium.2.2. Case 2

Figure 2. A 48-year-old female with intermittent chest pain, exrtional dyspnea, sinus tachycardia and moderate pulmonary hypertension. A and B, Diffuse thyroid enlargement (diameter of thyroid lobe = 27 mm and isthmic portion = 12 mm) (arrowheads on A and B) with a decreased attenuation (HU = 57) was noted on non-enhanced chest CT (B). C, Mild thymic hyperplasia (arrowheads) and mild enlargement of main pulmonary artery (32 mm) (arrow) are demonstrated on contrast enhanced chest CT, at the level of bifurcation of the main pulmonary artery. D, Mild thymic hyperplasia (39 × 47 × 13 mm) (arrowheads) is evident on a coronal CT image. E, Enlargement of the left (44 mm) and right atrium (54 mm) is demonstrated on contrast enhanced chest CT, at the level of the left atrium.

3. Discussion

There is no prior report in the English literature regarding specific MDCT findings of hyperthyroid CMP. In our cases, diagnostic indicators of hyperthyroid CMP on CT were a combination of the following: diffuse thyroid enlargement with decreased attenuation on non-enhanced chest CT (i.e., loss of normal iodine-based high attenuation of thyroid glands), thymic hyperplasia and cardiomegaly [i.e., predominantly right heart failure (enlargement of main pulmonary artery, right atrium and right ventricle) and left atrial enlargement]. We believe that the combination of the above CT findings is suggestive for the diagnosis of hyperthyroid CMP.

The previous studies reported that decrease in the attenuation of the thyroid glands on non-enhanced CT scan may be related to changes in the metabolism of thyroid hormone in patients with Graves’ disease (6, 7). Specifically, this may be caused by the fact that in patients with Graves’ disease, once iodine is taken up from the blood stream and synthesized to thyroid hormone, iodine is secreted again, without entering the thyroid pool (6). In a previous study, CT density of thyroid gland on the non-enhanced CT in patients with normal thyroid function (122 ± 18 HU; the lowest value = 87 HU) was significantly different from that of Grave’s disease (85 ± 22 HU, P < 0.001) (6). On average, 52.7% of patients with Grave’s disease had a CT density of less than 87 HU in the study. Therefore, loss of normal iodine-based high attenuation in the thyroid glands is expected to be quite specific, even though it is not sensitive as a finding for the detection of hyperthyroidism caused by Graves’ disease. Further research is required to address this issue.

Thymic hyperplasia is an additional CT indicator of hyperthyroid CMP in patients with hyperthyroidism (8-10). Thyroid hormone may have a direct effect on the thymus, leading to hyperplasia of thymic cells (8). However, the percentage of thymic hyperplasia on CT, occurring in patients with Grave’s disease, has not been elucidated yet and requires further study.

As for the third important CT feature of hyperthyroid CMP, previous studies have indicated that pulmonary hypertension is frequent in patients with hyperthyroidism (11-13). Consequently, hyperthyroid CMP is characterized by CT findings of pulmonary hypertension and predominantly right heart failure (i.e., enlargement of the right atrium, right ventricle, and main pulmonary artery on CT). Several possible mechanisms may account for hyperthyroid-induced pulmonary hypertension: first, thyroid hormones may cause increase in pulmonary vascular resistance, based on a direct effect on pulmonary vasculature (10); second, presence of atrial fibrillation and increase in blood volume in patients with hyperthyroidism may result in left atrial enlargement. The increase in left atrial pressure leads to the elevation of pulmonary capillary pressure, and, secondarily, an increase in pulmonary arterial pressure. Third, the left ventricular hypertrophy and reduced left ventricular contractile reserve may cause impaired left ventricular filling, resulting in increased left atrial pressure (4). Therefore, hyperthyroid CMP should be suspected in patients with unexplained pulmonary hypertension, as described in Case 2. Relatively rapid improvement of pulmonary hypertension after administration of anti-thyroid agent, as in Case 1, may be due to alleviation of atrial fibrillation and decreased pulmonary arterial resistance.

In conclusion, diagnosis of hyperthyroid CMP is possible based on recognition of certain MDCT findings. Familiarity with CT features of hyperthyroid CMP can lead to prompt diagnosis, early treatment and a better clinical outcome.